CPM has been proposed by Open Mobile Aillance (OMA) in 2005 to realize the intercommunication of multiple message services and provide uniform message services and perfect, consistent and convenient message service experience. CPM merges multiple existing message services including an Instant Message (IM), a Push-To-Talk Over Cellular (POC), a Mobile E-Email (MEM), a short messaging service (SMS), and a Multimedia Messaging Service (MMS) and the like.
OMA is currently formulating a CPM V1.0 standard. Service forms of CPM include: a CPM message based on a pager mode, a large message mode CPM message based on a session mode, and a session-based CPM session service. Before using such services, a user needs to register on and log into a CPM system, that is, to be ‘online’.
A Session Initiation Protocol (SIP) message is used to transmit a pager-mode message between CPM clients at CPM recipient and CPM sender sides and a CPM enabler. An SIP MESSSAGE cannot be used by a sender terminal to send a large message mode CPM message with a size beyond a limit (e.g. 1300 bytes).
A CPM service supports a CPM pre-defined group and a temporary group (CPM Ad-hoc group), a CPM subscriber can send a CPM paging message to a CPM Ad-hoc group or CPM pre-defined group, however, a message with a limit-beyond size cannot be included by an SIP MESSSAGE.
In existing specifications, there has been proposed no technical solution to the transmission of a large message mode CPM message to the terminal of an unavailable subscriber recipient via a non-CPM system in the case where a CPM subscriber sends a large message mode CPM message to a target CPM subscriber which is unavailable (offline or currently having no communication willing) or to a group in which one or multiple unavailable subscribers exist.